The vast majority of high voltage transmission conductors used includes strands of high strength steel surrounded by multiple strands of aluminum wire. The steel strands are the principle load bearing component holding up the wire, while the softer, more elastic aluminum strands include the majority of the electrical power transport component. Many variations of transmission wire operating at between approximately 115 kv to 800 kV revolve around this basic design concept, but all have these two basic components.
More recently, a single carbon fiber based pultruded composite core with an epoxy and/or fiberglass protective coating has emerged as a substitute for the steel support stranding in high voltage transmission conductors. Its attraction stems from three comparative broad areas. First, the carbon fiber based pultruded composite core has a proportionally higher tensile strength than commonly used high strength steel. Second, there is substantially less physical expansion/contraction properties when subjected to temperature changes than steel. Third, there is a higher tensile strength to weight ratio than steel resulting in a smaller overall diameter with higher strength.
The single carbon fiber based pultruded composite core results in an equal or greater tensile strength that can be achieved with a smaller overall diameter of carbon based pultruded rod than its high strength steel counterpart. Additional aluminum strands can then be added to the conductor, allowing more electrical load to be carried on the line. Because the carbon composite core has a dramatically lower coefficient of expansion than steel under high current load and/or high temperatures, the composite strands will not cause the conductor to “sag” (droop) between towers. When existing steel high voltage transmission lines sag, contact with the ground foliage can occur resulting in outages.
Carbon based, or “composite core conductor” as it is commonly referred to, is a viable alternative in re-conductor applications where existing capacity can be increased by replacing existing transmission conductors with steel wires with new composite based ones. Exemplary composite core conductors are disclosed in U.S. Pat. Nos. 7,041,909, 7,179,522 and 7,368,162 to Hiel et al., each of which is hereby incorporated by reference in its entirety.
Existing composite core conductors have a single pultruded core with a coating thereon serving as the sole structural support for the conductor. Single core design composite supported conductors may be at a higher risk of failure if a portion of the core is damaged accidently or intentionally. It is not uncommon to have rifle bullets from hunters either accidentally or intentionally strike and damage conductors. Damaging a portion of the single unitized composite core design can put the entire composite core rod structure at risk under a tensile load. Multiple stranded supported transmission conductors have a much higher reliability and survivability than single strand supported conductors of comparable tensile strength. Higher safety and reliability margins in tensile requirements can be achieved economically by adding strands to the design.
Accordingly, a need exists for a smaller diameter carbon based conductor support structure that is bundled together to replace the single unitized core conductor. Multiple stranding reduces the probability of total failure should one or more of the multiple strands become damaged. The integrity of the remaining strands with full tensile capability provides a higher probability of remaining intact and preventing the transmission wire from separating and falling to the ground.